Regiochemically well-defined fluorenone-alkylthiophene copolymers: Synthesis, spectroscopic characterization, and their postfunctionalization with oligoaniline

A new solution processable, regioregular, alternate copolymer of fluorenone and dialkyl-bithiophene, namely poly [(5,5'-(3,3'-di-n-octyl-2,2'-bithiophene))-alt-(2,7-fluoren-9-one)] (abbreviated as PDOBTF), was synthesized by three different preparation methods: chemical or electrochemical oxidation of 2,7-bis(4-octylthien-2-yl)-fluoren-9-one or polycondensation of 2,7-bis(5-bromo-4-octylthien-2-yl)-fluoren-9-one in the presence of Ni(0) reagent. Independent of the preparation method, the crude product is a mixture of high molecular weight fractions and short oligomers. It can be however easily fractionated into fractions differing in their molecular weight by sequential extractions with a series of solvents. The principal absorption band registered for the undoped polymer (lambda(max) = 384 nm for the THF solution and 389 nm for the solid state) originates from the pi-->pi* transition of the conjugated backbone and is blue-shifted because of the chain torsion effects caused by steric hindrance. This band is accompanied by a peak of smaller intensity (lambda(max) = 476 nm for the THF solution and 485 nm for the solid state) attributed to the n-pi* transition in the carbonyl group of the fluoren-9-one subunit. Preliminary photoluminescence studies show that PDOBTF exhibits a very large Stokes shift and emits red light (lambda(max) = 631 nm in THF solution and 643 nm in the solid state). Upon chemical p-type doping with FeCl4-, the polymer reaches the conductivity of sigma(dc) = 0.05 S cm(-1). Mossbauer spectroscopy studies of the doping process show that both structural subunits, i.e., the bithiophene subunit and the fluoren-9-one one, participate in the doping. PDOBTF can be relatively easily postfunctionalized by grafting aniline oligomers as pendant groups via the carbonyl groups of the fluoren-9-one subunit. By consequence, the spectrum of the modified polymer can be precisely tuned in the visible region by changing the grafting level.